Priority Mail packaging provided by the USPS must be used only for Priority Mail. Regardless of how the packaging is reconfigured or how markings may be obliterated, any matter mailed in USPS-produced Priority Mail packaging is charged the appropriate Priority Mail price. Any matter mailed in USPS-produced Critical Mail flat-size packaging will be charged Critical Mail flats prices only if all applicable standards in 323 are met; otherwise such matter will be charged the Priority Mail Commercial Plus Flat Rate Envelope price.

When mailing 200 or more Critical Mail flats in one mailing, prepare Critical Mail flats in USPS-provided flats trays with green lids, place the mail with addresses facing upward in the same direction. Place pieces in trays to maintain their orientation. The weight of a tray and its contents must not exceed 70 pounds. Cover each tray with the green side of the lid facing up and secure the lid with two straps placed tightly around the width of the tray. Label trays under the applicable flat tray label standards in 335.4.0 and as follows:

a. Line 1: Use L201; for mail originating in ZIP Code areas in Column A, use "MXD" followed by city, state, and 3-digit ZIP Code prefix in Column C (use "MXD" instead of "OMX" in the destination line and ignore Column B).